When comparing Polymer vs Flutter, the Slant community recommends Flutter for most people. In the question“What are the best frameworks for developing cross-platform mobile apps with JavaScript?”Flutter is ranked 5th while Polymer is ranked 12th.

Pros

Pro

Based on web components

Web Components are a collection of specifications released by W3C as a way to reduce the complexity of web apps by creating reusable components. Browser support is currently poor for web components, however Polymer is developed to make web components compatible with modern browsers.

Pro

Various basic components

It provides a base component.

Pro

HTML markup is not string

HTML markup as it can be a non-string.

Pro

CSS is easy to apply

CSS can be applied far more comfortably than React.

Pro

Flex layout components

It provides Flex layout components.

Pro

Excellent documentation

Polymer guides you as with tools (cli, build environment, app templates,..) as with complimentary documentation on all phases of app development from creation of app as progresive web app to production deployment instructions.

As Polymer is standards based, the whole community around those standards also helping in documentation and support.

Pro

Complete web app stack support

In addition to waste set of mature web components in Polymer Elements along with Vaadin Elements there are thousands of web components in the wild comparable to jQuery plugins set.

Pro

API is easy to understand, based on standard

The Polymer APIs are split on application layers and follow standards on all possible ways: Web Components, CSS variables, async API via Promises and so on.

Pro

Excellent routing

The router is embedded into CLI for project creation and covers as web as Progressive web app, also fused with Polymer layouts out of the box. The shop template for CLI has a complete solution including the routing.

Pro

No need for special debugging tools

The presence od specialized debugging tools are advertised by competitors. The all features of web components are natively supported by browser embedded development tools.

Pro

Great developer tools debug/hotreload/analyser

Pro

Based on Dart language

Dart is a Java like language, easy to learn and startup fast for millions of Java developers.

Pro

Open source

Pro

A single codebase for iOS AND Android

Pro

Fast

The developer's goal is to allow people to make apps running at 120 FPS.

Cons

Con

No server-side rendering

Polymer does not support server-side rendering. This results in higher loading times, more HTTP requests and it's not very SEO friendly, since search engines have no way of indexing a page if it's not rendered in the server.

Con

Still in development

According to the website, Flutter is still in its early stages of development.

Con

Dart is unpopular and never gained serious community traction like Kotlin or Java

Con

Does not support 32-bit iOS devices

If you plan on targeting iPhone 5, 5C or earlier, you can forget about Flutter.